1. Field of the Invention
The present invention relates to an optical fiber cable, more particularly, an optical fiber cable coated with a conductive metal coating, and a process for making same.
2. Description of the Related Art
Optical fiber cables have been extensively employed in a variety of fields, and when such optical fiber cables are employed in nuclear power plants or chemical plants in particular, a tolerance to heat is required, and thus optical fiber cables coated with layers of metal are used.
As one prior art process for making a metal coating for the optical fiber, a dipping method is known in which a threaded optical fiber made of silica based glass is dipped in a bath containing an aluminium, to thereby form an aluminium coating on the optical fiber. This dipping method, however, suffers from a disadvantage of a generation of a microbent, which causes an increase of a transmission loss in the optical fiber cable. More specifically, a coefficient of linear expansion of the silica based glass is approximately 0.4.times.10.sup.-6 /.degree.C., but a coefficient of linear expansion of aluminium is approximately 29.times.10.sup.-6 /.degree.C., i.e., approximately 70 times that of the silica based glass optical fiber. Further, the melting point temperature of the aluminium is high, and accordingly, when aluminium is cooled to become a solid and the aluminium coating is to be shrunk, a stress due to the aluminium coating causes a shrinkage of the optical fiber, to thereby cause the microbent. This microbent causes a transmission loss in the optical fiber. This problem of the dipping method also arises with metals other than aluminium.
Tanaka, et. al., in a paper, "Reducing Loss of Metal Coated Fiber", National Convention Record, The Institute of Electronics, Information and Communication Engineers, 1985, page 4-207, disclose a technology whereby an additional stress corresponding to a stress causing the microbent during the cooling of the dip material, such as aluminium, is applied to an optical fiber to compensate the microbent. It is known through experiments that, when the optical fiber coated with aluminium is expanded by the stress, e.g., by approximately 0.15%, the transmission loss is at a minimum.
Nevertheless, the compensation method disclosed in the above suffers from a disadvantage in that adding the additional stress increases the processes, and although the transmission loss can be compensated by applying the additional stress, the transmission loss can not be restored to an initial value. Further, the application of the additional stress must be held within a suitable range, because if an extra additional stress is applied thereto, the transmission loss is increased, and accordingly, the control of the application of this additional stress is difficult.
Further, when a metal coating is directly formed on a silica based glass optical fiber by the dipping method, the contact between the silica based glass optical fiber and the metal is weak, and therefore, when the optical fiber coated with the metal coating is bent, the metal coating is easily peeled from the optical fiber.
As an alternative to the above mentioned dipping method, Japanese Unexamined (Kokai) Patent Publication No. 51(1976)-54445 discloses a method of forming a metal coating by applying an electroless (chemical) plating process on an optical fiber. The electroless plating is carried out at a relatively low temperature, and thus a microbent will not be caused. The forming of the metal coating by the electroless plating process, however, suffers from a disadvantage of a low metal formation speed when forming the metal coating to a predetermined thickness required as a coating, and thus the optical fiber must be dipped into an electroless plating bath for a long time. The latter causes an incursion of hydrogen and/or water to the optical fiber, which causes a transmission loss of the optical fiber and/or reduces the mechanical strength of the optical fiber.
Further, U.S. Pat. No. 4,183,621 and EPC Publication No. 0308143 disclose methods wherein a carbon is coated on an optical fiber to increase a sealing thereof against an impregnation of hydrogen and/or water, to thereby prevent an incursion of hydrogen and/or water, whereby an increase of the transmission loss of the optical fiber is prevented. Also, a coefficient of linear expansion of carbon is close to that of the silica based glass optical fiber, and thus the microbent is not caused. The methods disclosed in the publications, however, are intended only to prevent the incursion of hydrogen and/or water, and are not intended to increase a tolerance to heat, a mechanical strength, and to have an electrical conduction function. As a result, optical fibers according to the methods disclosed in the publications can not be employed in high temperature conditions, or under conditions which requires a mechanical strength and an electrical conduction.